U.S. Pat. No. 1,612,427 employs electric waves for signaling approaching vehicles of the presence of another vehicle. The transmitting device is located on the vehicles.
U.S. Pat. No. 3,412,378 employs an electronic warning device, wherein the transmitter unit is located on the train.
U.S. Pat. No. 3,646,508 employs an optical and radio frequency signal to warn approaching motor vehicles of a disabled vehicle.
These aforementioned patents do not provide a means for stopping a vehicle approaching a railroad crossing or employs a transmitting device which communicates with the tracks.